Although a considerable body of research has focused on the neural substrates of social separation, relatively little is known about the neurobiology of social attachment. This proposal focuses on a monogamous rodent, the prairie vole (Microtus ochrogaster), which is distinguished by the formation of selective, enduring social attachments or pair bonds. Previous studies have demonstrated the importance of mating for the development of pair bonds in this species. Several lines of evidence implicate central vasopressin (AVP) pathways in the process by which mating facilitates prairie vole social attachment. AVP increases and AVP antagonists decrease mating-induced attachment in male prairie voles as measured by the development of partner preferences, selective aggression to strangers, and paternal behavior to offspring. This study will focus on the mechanisms by which AVP and AVP pathways are altered by mating in the prairie vole. The strategy is to compare monogamous prairie voles with non-monogamous montane voles (M. montanus), which show similar patterns of mating behavior but fail to form enduring, selective social attachments. The first study will investigate the effects of mating on central AVP activity in both species by examining central AVP immunoreactivity and content, mRNA expression, receptor distribution, and receptor coupling to second messenger. A second study will localize specific AVP pathways critical for social attachment in male prairie voles by using site-specific injections of an AVP antagonist which previously blocked pair bonding when given intracerebroventricularly. A third study will determine if increased central AVP release via non- social physiological stimuli will facilitate social attachment in male prairie voles in the absence of mating. Finally, a fourth study will investigate the role of AVP in the behavior of non-monogamous montane voles. Taken together, these studies should provide a comprehensive understanding of the role of central vasopressin pathways in the process of social attachment. By adopting a comparative approach, these studies may not only reveal a proximate mechanism by which mating facilitates pair bonding, but also suggest how a specific neuroendocrine system has been adapted for quite different behavioral functions in closely related species.